The conversion of transforming growth factor β (TGF-β) from a tumor suppressor to a tumor promoter occurs frequently during mammary tumorigenesis, yet the molecular mechanisms underlying this phenomenon remain undefined. We show herein that TGF-β repressed nuclear factor-κB (NF-κB) activity in normal NMuMG cells, but activated this transcription factor in their malignant counterparts, 4T1 cells, by inducing assembly of TGF-β–activated kinase 1 (TAK1)–binding protein 1 (TAB1):IκB kinase β (IKKβ) complexes, which led to the stimulation of a TAK1:IKKβ:p65 pathway. TAB1:IKKβ complexes could only be detected in NMuMG cells following their induction of epithelial-mesenchymal transition (EMT), which, on TGF-β treatment, activated NF-κB. Expression of a truncated TAB1 mutant [i.e., TAB1(411)] reduced basal and TGF-β–mediated NF-κB activation in NMuMG cells driven to undergo EMT by TGF-β and in 4T1 cells stimulated by TGF-β. TAB1(411) expression also inhibited TGF-β–stimulated tumor necrosis factor-α and cyclooxygenase-2 expression in 4T1 cells. Additionally, the ability of human MCF10A-CA1a breast cancer cells to undergo invasion in response to TGF-β absolutely required the activities of TAK1 and NF-κB. Moreover, small interfering RNA–mediated TAK1 deficiency restored the cytostatic activity of TGF-β in MCF10A-CA1a cells. Finally, expression of truncated TAB1(411) dramatically reduced the growth of 4T1 breast cancers in syngeneic BALB/c, as well as in nude mice, suggesting a potentially important role of NF-κB in regulating innate immunity by TGF-β. Collectively, our findings have defined a novel TAB1:TAK1:IKKβ:NF-κB signaling axis that forms aberrantly in breast cancer cells and, consequently, enables oncogenic signaling by TGF-β. [Cancer Res 2008;68(5):1462–70]